1. Field of the Invention
The present invention relates to an insulated radiating antenna, adapted to the conversion of analog signals into digital signals or vice versa.
It can be used especially in the field of radio communications, for example, as a sender or receiver antenna:
reception antennas for surface buildings capable of feeding a large number of receivers,
An elementary antenna of an antenna array of a detection and listening system with highly efficient anti-jamming protection;
An elementary antenna of an antenna array of a high-resolution direction-finding system.
More generally, it can be applied to all reception systems in which the amplitude and the phase of the useful signals have to be known with high precision. Systems such as this are for example detection and listening systems capable of cancelling out jamming devices, or high-resolution direction-finding or localization systems or again single-antenna multi-receiver reception systems with wide dynamic range.
2. Description of the Prior Art
In the field of radio communications, for example communications comprising listening, detection and localization means, the usual reception systems consist chiefly of the following elements:
An antenna 1, whose role is to pick up an incident electromagnetic wave and convert it into an electrical signal to be given to the receiver;
A receiver 3 used to select and insulate the signals known as useful signals,
A processing unit 2 that shapes the useful signals to be interpreted by the operator. In certain systems, the processing unit forms an integral part of the receiver.
FIG. 2 shows an exemplary prior art connection diagram of the connection between an antenna and a receiver.
The signals picked up by the antenna 1 are transferred to the receiver 3 by an electrically conductive feeder cable 4 which may be a bifilar type of cable or, more usually, a coaxial type of cable. When no precautions are taken during the installation of the antenna and of the cable, unwanted phenomena may appear and disturb the efficient operation of the reception systems, for example by modifying the pick-up capacity of the antenna, or again by introducing an undesirable phase shift in the useful signal to be processed. This defect is all the more pronounced as the system works in a wide range of frequencies, for example in HF (high frequency) reception systems covering the 1.5 MHz-30 MHz range, or as the system uses antennas which are small in relation to the wavelength, or as the antenna is installed at a great distance from the receiver.
Various approaches have been disclosed in the prior art to overcome phase shift defects if any. Some of these approaches are given in FIGS. 3 to 4.
FIG. 3a, shows an exemplary dipole antenna where the radio communications receivers 3 have asymmetrical structures. The input of a receiver has a terminal known as a hot terminal 3a and a cold or ground terminal 3b. This is also the case for the coaxial type feeder cables in which the first end 5a of the core has to be connected to the hot terminal 3a and the corresponding first end 5b of the shielding has to be connected to the ground 3b at one of its ends. This is true also for a frame antenna shown in FIG. 3b. Now, the structure presented by the majority of the antennas 1, as can be seen in FIGS. 3a and 3b, is rather a symmetrical structure comprising a pole 1xe2x80x2 and a pole 1xe2x80x3.
In this case, if the second ends 6a, 6b of the coaxial cable 4 are connected without precautions to one of the antennas 1 of FIG. 3a or FIG. 3b, taken for example by connecting the end 6a of the core to the pole 1xe2x80x2 and the end 6b of the shielding to the pole 1xe2x80x3 of the antenna, the incident electromagnetic wave to be picked up by the antenna will induce a current Ig known as the  less than  less than sheathing current  greater than  greater than  on the external skin of the shielding of the coaxial cable, which is added to the current Ia generated by the antenna itself. By Kirchoff""s law, the current at the pole 1xe2x80x3 of the antenna 1 is equal to the current Ia whereas, at the pole 1xe2x80x2, the current is equal to the sum of the currents Ig and Ia. It is then necessary to equalize the currents at the two poles 1xe2x80x2 and 1xe2x80x3 of the antenna to cancel out the sheathing current Ig of the coaxial cable 4 and hence the pick-up capacity of this cable. This symmetrization is obtained for example by means of an adapted device known as a xe2x80x9cbalunxe2x80x9d by those skilled in the art. This balun is placed between the coaxial cable and the antenna.
Furthermore, when the antenna is at a great distance from the receiver, the sheath current Ig, even when it is cancelled out at the antenna by the use of a balun, may be substantial in the case of a very long coaxial cable with imperfect shielding, for example a flexible coaxial cable whose shielding consists of a metal braid. This current then induces a parasitic electrical voltage between the core and the shielding of the coaxial cable and this voltage is found naturally at the input of the receiver. This parasitic voltage is proportional to the sheathing current Ig and to the physical length of the coaxial cable. The coefficient of proportionality is an intrinsic characteristic of the coaxial cable used, and is called a xe2x80x9ctransfer impedancexe2x80x9d. To overcome this defect, it is necessary to use cables with very low transfer impedance, such as double-braid cables, full-shielded rigid cables that have the drawbacks, in particular, of being costly, cumbersome and subject to constraints
In applications using a large number of reception antennas located in one and the same constricted area, for example a ship""s mast structure, there are problems of proximity owing to the supply cables. FIG. 4 gives a schematic view of two dipole antennas 1 and I, located one on top of the other for lack of sideways space. It can be seen that the cable 4 of the top antenna 1 masks the radiation of the bottom antenna I to some extent.
The object of the invention relates to an antenna used especially to prevent the above-mentioned effects by isolating it from its environment.
The idea of the invention lies especially in providing an antenna with means adapted to converting the picked-up analog signals into digital signals or digital signals to be sent into analog signals and in having these means available in a part that is insulated from electromagnetic waves and from all disturbing phenomena.
In the case of a receiver antenna, the means for transmitting the signals are chosen so as to transmit the digital signals, generated by the antenna that is the object of this invention, to the receiver with a sufficient bit rate dictated by the application and without making use of links based on electrical conductors that may disturb the operation of the antenna, at least with regard to the essential links between the antenna and the receiver.
An object of the invention is a dipole antenna with a radiating structure comprising at least one device adapted to the conversion of analog signals into digital signals and/or digital signals into analog signals, said device being positioned in a part of the antenna that is insulated from electromagnetic waves or phenomena.
According to one embodiment, the conversion device comprises, for example, an amplifier stage or an impedance-matching device adapted to the antenna and to the analog-digital converter.
According to a second embodiment, the conversion device may comprise a power stage and a digital-analog converter.
The antenna comprises, for example, a data transmission device integrated into the insulated part, this device possibly being an electrical converter linked with an insulating optic fiber that is transparent to electromagnetic waves.
An object of the invention is also a signal transmission and reception system comprising one or more antennas according to one of the characteristics mentioned here above, each antenna being connected by electrically non-conductive transmission means to at least one receiver.
The antenna comprising one of the above-mentioned characteristics is used for example in radiocommunications systems working in the HF frequency band ranging from 1.5 to 30 MHz.
The antenna according to the invention has the following advantages in particular:
it eliminates the disturbances caused in the operation by the presence of a xe2x80x9cmainxe2x80x9d link based on electrical conductors between the antenna and the receiver, for example the coaxial cables that are commonly used,
it can be easily integrated in terms of choice of position, the links between the antenna and the receiver being transparent to electromagnetic waves,
the antenna elements with galvanic insulation thus constituted have no electrically conductive xe2x80x9cmainxe2x80x9d link either with the electrical ground or with the mechanical ground of the radiocommunications system with which it is connected,
it can be used to work in a wide range of frequencies, wider than that commonly used in the prior art.